Plant Adaptations to the Environment

Use a cost- benefit analogy to explain seed size. Many adaptations are associated with “trade-offs” that may limit the degree of adaptation Use a cost- benefit analogy to explain seed size. This illustrates phenotypic variability of seed mass in a population growing on a dune near lake michigan. Even within an individual plant, seed size can vary alot Seed size vs. seed number Larger seeds have more endosperm for faster growth More seeds can be produced if they are smaller

Hypothetical model for seed fitness vs seed size Above a certain size, diminishing returns Below a certain size, no survival value

Morphological adaptations Adaptations to life on land Photosynthesis developed in oceans; land plants had to cope with desiccation. Cuticle: waxy covering over epidermal cells Vascular tissues: xylem and phloem Pollination by wind in dry conditions Seeds with seed coat and endosperm

Morphological adaptations Growth forms Wide variety of growth forms and architectures have evolved to adapt to different light, moisture, temperature conditions The meristem is undifferentiated tissue that produces new growth; in the embryo of a seed, or in terminal buds, lateral buds, the cambium and elsewhere in perennial plants

9/10/07

Raunkiaer’s classification of perennial plant growth forms based on location of meristem relative to soil surface 9/10/07

Morphological Adaptations Leaf Morphology Size: Smaller in arid environments, larger and thinner in forest environments. Why? Pubescence on leaf surfaces is found in hot/dry, and cold environments. Why?

Dispersal is fundamental for species survival Adaptations for seed dispersal Many adaptations exist to ensure cross-fertilization (pollination) 9/10/07

Longevity ANNUALS Adaptive where probability of an adult surviving an unfavorable season is low Germination may be triggered by rain, light, smoke, heat, cold BIENNIALS Live for 2 or more years before flowering and then dying (semelparous) PERENNIALS Monocarpic—reproduce once, then die (semelparous) Polycarpic—reproduce repeatedly (iteroparous) Mast years, to reduce seed predation

Phenology EPHEMERAL PLANTS DECIDUOUS PLANTS Avoid periods during the year with environmental stresses Take advantage of short, favorable periods with fast growth DECIDUOUS PLANTS Avoid stressful periods by shedding leaves Leaf growth and photosynthetic rates are high Considered more “expensive” than evergreen leaves in terms of nutrient use High nutrient cycling is required to support deciduous leaves Define phenology? timing of growth and reproduction within a season or year; mainly constrained abiotically by temperature or moisture

Phenology EVERGREEN PLANTS Tolerate stressful periods with leaves that can withstand cold or drought Leaves may live <1 to >20 years Leaf growth and photosynthetic rates are low but can occur over wider range of conditions Evergreen leaves cost about the same amount of energy as deciduous leaves, because lignin, fiber, wax are expensive to make Adapted to tolerate lower nutrient status and slower cycling EVERGREEN PLANTS

MacArthur & Wilson’s r vs. K selection Opportunistic vs. climax species r-selected traits (favored at low pop’n density) Fast growth and reproduction Poor competitors K-selected traits (favored at high density) Slow growth, delayed reproduction Density dependent populations Most species fall in between these extremes This approach suggests that natural selection works on populations rather than individuals Logistic model: dN/dt = rN (K – N)/K R=intrinsic growth rate K=carrying capacity In reality, there is no reason that evolution would require a tradeoff between r and K traits; both will increase under logistic growth.

Insight into trade-offs resulting from natural selection on certain traits 9/10/07